Fix GC.measure_total_time regression

[rianmcguire]

Commit 93ac740 introduced a regression where measurements would still be taken after setting GC.measure_total_time = false.

This PR re-introduces the checks for the objspace->flags.measure_gc flag around the measurement code.

Fixes [Bug #20157]

[KJTsanaktsidis]

Thanks for your fix - it looks correct to me. Could you add your reproduction script from the issue you opened as a testcase (probably in test/ruby/test_gc.rb maybe?). It should more or less work as-is in an assert_separately block I think.

[KJTsanaktsidis]

(just the check on GC.stat(:time), asserting on what syscalls are made is obviously going to send the test directly to flaketown)

[rianmcguire]

CI failures on the latest commit look like unrelated flake